Many industries have machinery and parts that are intermittently or continuously exposed to corrosive environments. Due to the nature of these environments, direct observation of the corroding parts is usually impossible since they tend to be in inaccessible areas. In most cases, actual measurements of corrosion levels require that entire systems be brought off line and opened up.
Bringing systems off line, however, can be very expensive and time consuming, therefore corrosion rates tend to be estimated. Since failure of some parts can be catastrophic, the corrosive levels of the parts are overestimated which results in the operation of the processes in which they are used are kept on the conservative side. This reduces the risk of a failure due to excessive corrosion, however by overestimating the corrosion levels, process efficiency is typically lost. Therefore, without a more accurate estimation of corrosion levels, efficiency needs to be sacrificed to prevent part failure.
An example of a such system that has a corrosive environment is shown in FIG. 1. This figure illustrates a heat recovery steam generator (HRSG), which is used to turn otherwise wasted hot gasses into useful steam. Hot gasses enter 2 the HRSG from sources such as a gas turbine (not shown). Depending on the nature of the fuel used in the gas turbine, the hot gasses will contain varying levels of corrosive substances. In a HRSG, the hot gasses pass over heat transfer surfaces made up of tubes in which water, under pressure is converted to steam. The steam rises in the tubes and is collected in a series of three drums, a high pressure (HP) drum 6, an intermediate pressure (IP) drum 8, and a low pressure (LP) drum 10. Ultimately, the hot gasses are vented 4 after being depleted of most of their useful heat.
In the type of HRSG represented in FIG. 1, the heat from the hot gasses are transferred to either water, steam, or a combination of water and steam through use a boiler tube, which is a type of heat exchanger. An example of a typical boiler tube 20 at the rear of the HRSG is shown in FIG. 2. In this figure, the hot gasses 24 first pass over evaporation tubes 12 that heat water contained in the tubes to the point where the water is converted partially into steam. This water-steam mixture enters the LP drum 10, where the steam is separated 11. In this process, the LP drum requires make up water 22 to replace the volume lost by conversion to steam 11 and other causes. In order to capture the greatest amount of heat from the now cooled exhaust gas, the coldest water entering the system is first heated by passing it through a boiler tube. The boiler tube 20 runs water 22 through a center passage, while the hot gasses 24 pass over the outer surfaces, usually comprised of fins 26, to transfer the heat from the gas to the water. The hot gasses 24, however, contain corrosive elements, such as sulfur, that will erode the boiler tube 20. If the boiler tube corrodes to the point of breach, the entire HRSG system becomes subject to damage and will need to be taken offline.
The boiler tubes that carrying water to the LP drum 10 of the HRSG are particularly susceptible to corrosion, since at lower temperatures, generally between 110–250° F. (43–121° C.), corrosive elements condense out of the hot gas and form acids. In fact the section of the boiler tube that first receives the make up water 28 is the most susceptible to corrosion because it is the coldest part of the boiler exposed to the hot gasses 24. The HRSG system has a life expectancy of about 25 years, and if a boiler tube fails before this time, replacement costs an enormous amount of time and money. To prevent failure of the boiler tube 20, the temperatures of the tubes carrying water to the LP drum 10 are kept higher than they may other wise be, so that less corrosive elements condense out of the hot gas, causing a higher temperature gas to be vented to the environment, which is a waste of heat energy as well as a source of heat pollution. Over the course of 25 years this represents a substantial loss of revenue to the operator, and the waste of energy and excess heat pollution can have a large negative effect on the environment.
What is needed is a method and apparatus that can monitor the corrosion levels of component parts in corrosive environments.